Principle Of Cryopump Operation - Ulvac CRYO-U Series Instruction Manual

CRYO-U Cryopump Instruction Manual
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Appendix D

PRINCIPLE OF CRYOPUMP OPERATION

Evacuation operation of the cryopump is accomplished by causing the gas molecule hit on a
surface (cryo surface) cooled to extremely low temperature to be condensed and absorbed on
the cryo surface.
Our CRYO-U ® series cryopump uses a helium refrigerator for cooling the cryo surface and clean
high vacuum and extra-high vacuum can be obtained by simple operation.
The cryo surface is cooled to 12K to 20K according to the heat load.
When the cryo surface is cooled to 20K, the vapor pressure of nitrogen condensed on this
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surface is less than 10 Pa (refer to Fig.D-1). Therefore, gas (oxygen and argon, etc.) whose
vapor pressure is lower than that of nitrogen is normally condensed on the cryo surface and
evacuated.
Vapor pressure of neon, hydrogen and helium is 0.1Pa or more even at 12K, and the CRYO-U ®
series cryopump cannot condense these gasses on the cryo surface. Such gasses are absorbed
and evacuated by the absorbent installed on the cryo surface.
Since the cryopump evacuates gas, as stated above, by condensing or absorbing the gas on the
cryo surface, evacuated gas is accumulated on the cryo surface. In the case of condensation,
when the quantity of the gas condensed on the cryo surface increases and the condensed layer
reaches certain thickness, surface temperature of the condensed layer rises and the condensing
speed of the gas molecules is lowered. In the case of absorption, when the quantity of the
absorbed gas approaches to the saturated absorbed quantity decided according to the
temperature of absorbent and the pressure of absorbed gas, absorbing speed of the gas
molecules is lowered considerably. Mainly due to these 2 factors, evacuation performance
(pumping speed and ultimate pressure) of the cryopump indicates remarkable reduction from
certain point with an increase in the quantity of the evacuated gas. Therefore, it is necessary to
set the temperature of the cryo surface to the room temperature periodically, vaporize the
condensed/absorbed gas components and exhaust them to the outside. This operation is called
"regeneration" by which the performance of the cryopump can be recovered through the
"temperature rise (exhaust of gas) -> degassing -> cooling" process.
MA0D001E,2010.01 ULVAC CRYOGENICS INCORPORATED D-1